The present invention relates to a method of setting color copying conditions in color image copying and more particularly to a method of setting color copying conditions when copying half-tone color images from color originals.
In copying color images from color originals, using color image copying machines such as electronic color image copiers, ink-jet color printers, laser color printers, photographic color printers using reversal color paper or diffusion transfer type photosensitive materials, video color image copiers apparatus, or heat sensitive copying machines, it is essential to reproduce a color image with both proper color density and proper color balance. To provide well finished color copies, color copying machines are generally adjusted to a proper color copying condition when starting operations and periodically thereafter. The important factors on which the color density and color reproductivity of color copies depend are the dispersion in performance of the copying lens systems, changes in the copying lens system due to aging, dispersion in characteristics of developers, developer changes due to aging, and changes in developing conditions. Therefore, color copying machines, even of the same type, set their color copying conditions individually. For example, in silver salt diffusion transfer processing type color copying machines, the optical factors are dispersion and the changes due to aging of the light source, the copying lens, color filters, reflection mirrors, light sensors, the scanning speed at which a color original is scanned, and the like. The factors which depend on photographic materials are the types of and the changes due to aging of the photosensitive materials, the differences in manufacturing batches, and the like. The factors which depend on developing materials include the characteristic dispersion and changes due to aging of developers. Furthermore, the factors which depend on developing conditions include the time of developing, the temperature of developing, and the like. In electrophotographic copying machines, other factors include the sensitivity dispersion and the changes due to aging of the photosensitive drums, the characteristic dispersion and changes due to aging of toners, and the characteristic dispersion and changes due to aging of chargers.
Heretofore, various systems for setting color copying conditions have been well known. One such method, as is disclosed in, for example, Japanese Patent Unexamined Publication No. 57-7051, comprises providing a standardized density pattern near a table on which a color original is placed, optically exposing a photosensitive drum to the standardized density pattern to form an electostatic latent image on the drum, detecting a surface voltage of the electrostatic latent image, and controlling the charge and exposure of the photosensitive drum in such a way as to equalize the detected surface voltage to a desired value.
In another system, as is described in Japanese Patent Unexamined Publication No. 57-60348, a standardized density pattern is disposed adjacent to the available area of a table on which a color original is placed. A toner image of the standardized density pattern is formed on a photosensitive drum and the density of the toner image is detected by means of a special densitometer. The charge and exposure of the photosensitive drum is controlled in such a way as to equalize the detected density to a desired value.
A third system is disclosed in Japanese Unexamined Patent Publication No. 61-14663, where, while controlling the luminance of a light source or developing a bias voltage for the developing device during copying, a black and white copy with strips having different densities is made. This black and white copy is visually examined to set the proper density settings.
As disclosed in Japanese Unexamined Patent Application No. 60-243649, an electrostatic latent image of a test chart is formed on a photosensitive drum while increasing or decreasing the exposure. A toner image of the electrostatic latent image formed on paper is visually examined to determine the proper exposure.
A problem associated with the above described systems is the difficulty in setting the copying conditions with high accuracy. For example, in the first system, it is difficult to set the color copying conditions in consideration of the developing conditions, the characteristic dispersion, and changes due to aging of the development materials. It is also difficult to set accurate color copying conditions because a corrected exposure is determined based upon the density of a single point, on the assumption that charging is proportional to exposure. Further, the use of this technique is limited to specific types of copying machines.
The second system described above allows setting the color copying conditions in consideration of the changes and dispersion of various factors, in that a special densitometer is employed to measure the density of the toner image. However, since measurement is effected at a single point, it is hard to set correct color copying conditions. It is also difficult to apply this system to silver salt type copying machines or color copying machines using diffusion transfer type photographic materials.
The remaining color copying systems require visual examination of a black and white copy in order to correct the exposure levels. High skill is required to correct the exposures for all three colors (blue, red, and green) to provide well balanced copies.